What is the area, in square units, of the interior region formed by the lines $y = 2x - 4, y = -3x +16$ and the $y$-axis?
Answer: To find the vertices of the triangle, we find where the two lines $y=2x-4$ and $y=-3x+16$ intersect. Solving $2x-4=-3x+16$, we get $x=4$.  Substituting $x=4$ back into $y=2x-4$, we find $y=4$.  Therefore, $(4,4)$ is one of the vertices of the triangle.  The other two vertices are the $y$-intercepts of the two lines, namely $(0,16)$ and $(0,-4)$.  Taking the side joining $(0,16)$ and $(0,-4)$ as the base of the triangle, we find that the area of the triangle is $\frac{1}{2}(\text{base})(\text{height})=\frac{1}{2}(16-(-4))(4)=\boxed{40}$ square units.

[asy]
unitsize(3mm);
defaultpen(linewidth(.7pt)+fontsize(8pt));
dotfactor=4;

fill((0,-4)--(0,16)--(4,4)--cycle,gray);

draw((-2,0)--(5,0),Arrows(4));
draw((0,-5)--(0,18),Arrows(4));

draw((0,-4)--(4,4));
draw((0,16)--(4,4)); [/asy]